1. Someone who does not mingle at a social event, such as a party, dance, etc.
2. A person or an organization that is forced to stay at the sidelines of some activity.

From wall, from Old English weall, from Latin vallum (rampart), from vallus (stake) + flower, from Old French flor (flower, flour, the best of anything). Earliest documented use: 1578.

TALL FLOWER - Helianthus annuus

CALLFLOWER - one of several vegetables in the species Brassica oleracea in the genus Brassica. Typically, only the head is eaten (the edible white flesh).

WALL GLOWER - 1. a nightlight; 2. chaperone at a teenagers' dance